What Size Inverter Do I Actually Need? A Room-by-Room Load Calculation

Most inverter sizing advice in Pakistan comes down to “just get a 5kW, it’s the popular one.” That answer is right for a lot of households by accident, and badly wrong for plenty of others who either overpay for capacity they never use or undersize and spend two years frustrated that the AC trips the unit every afternoon.

The actual method is not complicated. You walk through the house room by room, list what runs in each one, add the wattages, and size around the total rather than around what your neighbour bought. This is that calculation, done properly, with real numbers rather than guesses.

For a sizing recommendation matched to your actual load rather than a general rule of thumb, WhatsApp 03192070022.

The Two Numbers That Actually Matter

Every appliance has two ratings, and confusing them is the single most common sizing mistake we see.

Running watts is what the appliance draws continuously once it is operating. This is the number printed on most spec sheets and the one people default to.

Surge watts is the short burst a motor-driven appliance pulls at the instant it starts, before settling down to its running figure. A refrigerator or AC compressor can surge to two to three times its running wattage for a second or two. An inverter sized only against running totals will trip or shut down the moment two motor loads start close together, even though its “continuous” rating looked comfortable on paper.

Size your inverter against the running total of everything that could realistically be on at once, then confirm its surge rating comfortably covers your single largest motor-driven appliance starting while everything else is already running.

Room-by-Room Load List

Living room

Typical living room total: roughly 150 to 250W running.

Bedrooms (per room)

A bedroom without AC runs 100 to 150W. A bedroom with a single AC running is the single biggest line item in most Pakistani homes, and it is where most sizing goes wrong. For the full detail on matching an inverter to one AC specifically, see our inverter for 1.5-ton AC guide.

Kitchen

The refrigerator’s surge figure surprises most people. A compressor that idles at 150W can briefly demand 1,000W or more at the moment it kicks on, and it does this unpredictably throughout the day and night, which is exactly why it needs to be counted in your surge planning even though its running draw looks small.

Kitchen, gas-free households

If cooking runs on electricity rather than gas, add an induction hob (1,500–2,000W running) and possibly an electric kettle (1,000–1,500W) or oven (1,200–2,500W). These push a kitchen’s load into commercial-appliance territory and deserve their own circuit thinking rather than being folded into the general household total.

Bathroom

Electric geysers are the appliance most households forget to count and the one most likely to blow a sizing calculation. If yours runs on gas, this line drops out almost entirely. If it is electric, it is often the single heaviest load in the house and usually deserves to be excluded from inverter backup altogether rather than sized around, since backing up a 3kW to 4.5kW geyser through an inverter and battery bank is rarely economical.

Utility and common areas

Worked Example: A 10-Marla Home

A typical 10-marla house running three bedrooms with one AC on at a time, a living room, kitchen with a gas stove, and a 0.5HP water pump, essential-only backup during an outage:

Add 20% to 25% headroom for future load growth and to avoid running the inverter at its absolute ceiling continuously, and you land at roughly 2,800W to 2,900W of required capacity, which comfortably fits a 3kW to 3.6kW inverter. Buying a 6kW unit for this exact load is not wrong, but it is capacity you are paying for and rarely using, unless you specifically plan to add a second AC or expand the house within the next year or two.

For a side-by-side look at exactly when the jump to a larger capacity earns its cost, our 5kW vs 6kW solar system comparison walks through that decision in detail.

Worked Example: A Home Running Two ACs Simultaneously

Same house, but both bedroom ACs running at once during a summer night, plus the kitchen and living room load:

With headroom, this pushes into the 4kW to 5kW class. This is the calculation most households skip, sizing around “one AC at a time” and then buying a second unit for another room the following summer, at which point the inverter they already own is undersized for the household’s actual pattern of use.

Where People Get the Math Wrong

Counting nameplate wattage instead of real running draw. A label often lists an appliance’s maximum rated wattage, not what it actually pulls under normal use. Inverter-technology ACs in particular run well below their nameplate figure most of the time.

Ignoring surge entirely. A running total that fits comfortably within an inverter’s continuous rating can still trip the unit if two motor-driven appliances start within the same second and their combined surge exceeds the inverter’s surge rating.

Forgetting the geyser. Electric water heaters are large enough to distort an entire calculation if included, and expensive enough to skip if you plan the backup around everything except hot water.

Sizing for today only. A new AC, a growing family, or a home office added in eighteen months changes the total. Building in 20% to 25% headroom now is cheaper than replacing the inverter later. Our what size inverter do I need guide covers this future-proofing question in more general terms if you want the broader version of this same decision.

Sizing the inverter without sizing the battery bank to match. A correctly sized inverter paired with too few batteries still leaves you in the dark within an hour. Our how many batteries for a 6kW inverter guide walks through that half of the equation.

Larger Homes, Shops and Multi-AC Households

The room-by-room method scales the same way for bigger properties, it just adds more line items. A one-kanal house running three or four ACs, a home office with two desktops, and a full kitchen easily crosses 5,000W to 6,500W running once you add the totals honestly rather than assuming a single popular capacity covers it.

A small retail shop follows the same logic with a different mix: display lighting, one or two split ACs, a small fridge or freezer for stock, a computer and printer at the counter, and sometimes a water dispenser. Shop owners frequently undercount lighting because individual bulbs look trivial, but a shop with twenty LED tubes and spotlights can add 300W to 500W that a household calculation would never encounter.

Once a property’s running total climbs past roughly 6,000W, or the installation needs three-phase power for balanced heavy motor loads, the sizing conversation shifts from a single-phase household inverter into the commercial and semi-commercial range. Our 12kW hybrid inverter price in Pakistan guide covers that jump in detail, including the single-phase versus three-phase decision that starts to matter at this scale.

The same headroom logic applies at every scale. A shop planning to add a second display fridge or a household expecting to finish an upper floor within two years should size 20% to 25% above today’s calculated total now, rather than replacing the inverter once the new load actually arrives.

A Simple Method You Can Do Yourself

List every room. Under each one, write down what actually runs there and its running wattage from the tables above or the appliance’s own label. Decide honestly what runs simultaneously during a typical outage, not everything you own at once, and add those running figures together. Add 20% to 25% for headroom. That number is your minimum inverter capacity. Separately, identify your single largest motor-driven appliance and confirm the inverter’s surge rating clears it with everything else already running.

If that math gets complicated, or you would rather not guess on a purchase this size, send us your room list and we will do the calculation for you at no cost.